1. Field of the Invention
The present invention relates to a method and an apparatus for recording or reproducing information in or from an optical disc and, more particularly, to a method and an apparatus for recording or reproducing information in or from an optical disc upon irradiation with a light beam, the optical disc having a recording surface with respective tracks divided into a plurality of sectors.
2. Description of the Prior Art
In a conventional method of recording/reproducing information in/from an optical disc, a laser beam is focused to form a small spot with a diameter of about 1 .mu.m on the rotating optical disk having a recording surface formed of photosensitive recording medium. Information signals are recorded in the recording surface at a high density in accordance with formation of grooves/projections, formation of pits, changes in reflectivity, and magnetization directions. The recorded information is reproduced as needed.
Optical disc reproduction apparatuses are exemplified by optical video disc players or digital audio disc (DAD) players. Optical disc recording/reproduction apparatuses are exemplified by so-called optical disc filing systems and the like. The recording signals include a picture signal, an acoustical signal, a digital signal and the like.
In an apparatus of this type, guide tracks are formed at a narrow pitch on the disc to allow optical detection of the tracks and high-density recording. The guide tracks are subjected to tracking servo control, so that information signal recording or information signal reproduction can be performed along or on the guide tracks.
The guide track is selected in accordance with the content of recording information and the type of signal to be recorded. The guide tracks are formed in a spiral or concentric manner with respect to the center of the disc. When the information has an analog form, the information can be generally controlled in units of tracks. On the other hand, when the information has a digital form and the length of data is not determined as in a video signal, information cannot be controlled in units of tracks. The tracks are often divided into sectors such as information blocks, so that information is controlled in accordance with the track and sector address data.
The guide tracks are generally formed in grooves and at the same time the track and sector addresses are formed of three-dimensional phase grooves. Another method is proposed wherein a refractive index of a photosensitive recording material constituting a flat recording surface is changed by using a high power light source prior to recording, and the guide tracks and track and sector address portions are formed in accordance with a change in reflectivity.
When a defect is present in a disc, an accurate tracking error signal cannot be obtained when the guide track is subjected to tracking servo control so as to perform recording/reproduction with respect to a single track. In this case, a track jump (i.e., an erroneous recording or reproduction is performed with respect to the guide track) occurs.
Conventionally in order to prevent such a track jump, a light spot which is about to scan the defective track is detected to generate a detection signal. A tracking error signal and a sample-held or sampled and held signal of a tracking error signal obtained immediately prior to scanning of the defective portion are switched in accordance with the detection signal, thereby driving a tracking actuator. This conventional method will be described in detail with reference to FIG. 1.
Referring to FIG. 1, a contact a of an analog switch 2 is set in a state shown in FIG. 1, and an output (i.e., a tracking error signal e.sub.N) from a tracking error detecting circuit 1 is supplied to a driver 3 through the analog switch 2. The tracking error signal e.sub.N is amplified by the driver 3 to a proper level, thereby driving a tracking actuator 4. The tracking error signal e.sub.N is always sample-held by a sample and hold circuit 5 at a proper timing. A sample-held tracking error signal e.sub.A from the sample and hold circuit 5 is connected to a contact b of the analog switch 2.
When a defect of the disc is detected by a defect detecting circuit 6, a switching signal S.sub.C is generated from the defect detecting circuit 6, and the analog switch 2 is switched from the contact a position to the contact b position. The sample-held tracking error signal e.sub.A thus drives the tracking actuator 4. At the same time, the switching signal S.sub.C is also supplied to the sample and hold circuit 5, so that the sample-hold operation is inhibited. The sampled tracking error signal immediately before the light spot reaches the defective portion is held while the light spot passes through the defective portion.
The defect detecting circuit 6 utilized the fact that an amount of light reflected by or transmitted through the disc in the presence of a defect greatly changes as compared with the case in the absence of a defect. The defect detecting circuit 6 generates the switching signal S.sub.C in accordance with the duration corresponding to the presence of the defect and its light amount level.
However, according to the conventional recording or reproduction method described with reference to FIG. 1, when a large eccentric state occurs in a disc while the actuator is driven in response to the sample-held tracking error signal, an objective lens is not held at a predetermined position along the radial direction of the disc, so proper tracking servo control is not performed. As a result, even after the light spot passes through the defective portion and the proper tracking signal is obtained, the tracking control does not start from the same track prior to defective portion scanning. In other words, a track jump occurs, resulting in inconvenience.